1. Field of the Invention
The invention generally relates to optical fibers that may be used in distributed temperature sensing (“DTS”) systems or other distributed sensing systems. More particularly, the invention relates to systems used to deploy such optical fibers in double-ended DTS or other distributed sensing systems and their combination with other sensing systems.
2. Description of Related Art
A system for remote sensing of temperature (and other parameters) that works on the basis of Raman or Brillouin scattering in an optical fiber operated in single-ended mode is known. In one embodiment of a DTS system, an interrogation (probe) laser pulse is launched down the optical fiber, which is deployed along a region of interest, such as a wellbore. The probe pulse is scattered at each point along the fiber length, generating two backscattered Raman signals of new wavelengths, these signals being the anti-Stokes signal and the Stokes signal. The strengths of the signals are temperature-dependent. The two backscattered signals traverse along the length of the multimode fiber to surface electronics where they are detected, and the ratio of their respective signal strength values is calculated in order to provide an estimate of the local temperature at the point of generation of the backscattered signals. However, the accuracy of this system is hindered by the signal propagation loss that occurs when the backscattered signals travel to the surface electronics, this problem being compounded by the loss usually being different for the respective return wavelengths of the backscattered signals. Thus, practically, the ratio of the signal strengths of the backscattered signals provides a measure of the local temperature at a point of interest plus the cumulative difference in the losses at the respective return wavelengths of the signals. This is the case with single-ended DTS applications.
Clearly, this system could be operated with improved accuracy if the differential loss of the backscattered signals were known. The effects of signal loss can be separated from those of temperature by double-ended operation, which entails looping the optical fiber back to the surface electronics and repeating the measurement from the opposite end of the fiber. As temperature changes tend to appear in the same sense when viewed from opposite fiber ends, whilst loss effects appear opposite in sense when viewed from either direction, these two parameters can be distinguished by combining the measurements obtained from the fiber ends.
However, double-ended DTS systems often require more space than single-ended systems. In applications where space is at a premium, such as in wellbores, it would be desirable to devise a way in which double-ended DTS systems could be deployed without requiring as much space as prior art double-ended systems.
Thus, there exists a continuing need for an arrangement and/or technique that addresses one or more of the problems that are stated above.